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Further Studies on the Chemotherapy of Adenocarcinoma 755 with 6-Thioguanine^*

( Life Sciences Research, Stanford Research Institute, Menlo Park, California)

When Ca-755 tumors of various sizes were treated with three doses of C¹⁴-labeled 6-thioguanine (10 mg/kg at 24-hour intervals) the specific activities of their deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) at the end of the treatment were independent of the starting size of the tumor. During the subsequent rapid tumor regression the specific activity of DNA and RNA decreased linearly with the amounts present, and when projected the plots cut the specific activity ordinates at values which again were independent of the original tumor size. A second course of thioguanine therapy, commenced before tumor recurrence, produced a very much higher proportion of "cures" than did either a single course of therapy or a second course given after tumor recurrence. A second course of therapy with C¹⁴-labeled thioguanine before recurrence showed that some of the drug was incorporated into the nucleic acid of the remaining tumor cells.

^* This work was supported in part by Contract No. SA-43-ph-3068 with the Cancer Chemotherapy National Service Center, National Cancer Institute, National Institutes of Health; and in part by United States Public Health Service Grant No. CY-4551.

Received 7/10/63.